Thermoplastic elastomers (TPEs) are block co-polymers made of a hard component and a soft component. The two components undergo microphase separation. The solid domains formed by the hard component are glassy or crystalline. The solid domains are dispersed within a matrix of the soft component and act as both physical crosslinkers and filler particles. The polymer chains of the soft component have a Tg below the service temperature and are able to stretch but are restricted from flowing by the solid domains formed by the hard component. In the absence of the hard component, the soft component can flow. TPE networks can be recycled, and can be prepared by liquid processing technologies.
In conventional TPEs, the molecular weights of the hard and soft components have to reach a relatively high value for effective microphase separation. Even after the above molecular weight requirements are satisfied, there is still a certain degree of mixing in the microdomains. The incomplete segregation often compromises the properties and causes problems such as plastic deformation. For block copolymer TPEs, where the block responsible for physical crosslink is usually a styrenic block, the service temperature cannot exceed the glass transition temperature of polystyrene (Tg=−90° C.), which significantly limits their usefulness.
Supramolecular TPEs are TPEs containing monodisperse crystallizable hard segments. Most supramolecular TPEs known to date are segmented TPEs with polyether or polyester soft components (Eisenbach, C. D.; Baumgartner, M.; Günter, C. in Advances in Elastomers and rubber elasticity, Eds. Lal, J. and Mark, J. E.; 1986, Plenum Press: New York). Compared to conventional TPEs, the supramolecular TPEs display complete phase separation, sharp melting temperature, low plastic deformation, and improved strength (Gaymans, R. J. Prog. Polym. Sci. 2011, 36, 713-748).
Some grafted copolymer thermoplastic elastomers (TPEs) are characterized by elastomeric chains that are physically crosslinked by the interaction of associating pendent groups. Supramolecular TPEs with the monodisperse hard segments grafted to the elastomer backbone have not met much success because they have not been shown to possess the strength of typical TPEs. If the molecular weight between crosslinks, which is the molecular weight of soft segment for segmented TPEs and the molecular weight between grafts (MG) for grafted TPEs, is too low, the material will behave with more plastic character than elastic, for example, a yield point will appear.
Stadler at el. examined polybutadiene grafted with 4-(3,5-di-oxo-1,2,4-triazolin-4-yl)benzoic acid (TZB) (Hilger, C.; Stadler, R. Makromol. Chem. 1990, 191, 1347-1361). The structure of TZB contains two hydrogen bonding sites; the carboxylic acid and the urazole. These two sites associate as urazole-urazole and acid-acid pairs to from an extended hydrogen bonded structure. The extended hydrogen bonded structures aggregate together to form a higher ordered bundle-like structure, which is phase separated from the polybutadiene segments. As the result of phase separation, the above TZB-grafted polybutadienes are elastic solids below the melting temperature of the TZB domains.
Stadler's study focused on tensile properties at low strain (<300%) and did not report the ultimate strength and strain. In this system, because TZB has a fixed molecular weight, increase of TZB content necessarily decreases the molecular weight between grafts, MG (i.e., TZB has to be densely grafted in order to achieve a certain high weight percent). Even when the weight percent of TZB is only 4%, MG is already too low. As a result, plastic yielding occurs as indicated by a yield point in the stress-strain curve. This means, although Stadler did not show because only low strain data were given, that a strong elastomer without plastic yielding cannot be obtained by TZB.
Recently, Hélène Montès et al prepared grafted polydimethysiloxane (PDMS) with monodisperse bis-ureas for the associating groups (Colombani, O.; Barioz, C.; Bouteiler, L.; Chanéac, C.; Fompérie, L.; Lortie, F.; Montès, H. Macromolecules 2005, 38, 1752-1759). The bis-urea segments phase separate to form crystalline domains.
The two grafted copolymers have very similar weight percentages of hard content, but differ in their total molecular weights. Polymer PDMS(13)-g-BuP has a low molecular weight of 10,500 g/mol. At this molecular weight this polymer is analogous to BuP/PDMS(4)/BuP in that the soft block molecular weight is too low for sufficient entanglement. The result is a weak material. The higher molecular weight BuP/PDMS(30)/BuP (31,000 g/mol) displays an ultimate strength (4 MPa) and ultimate strain (300%) that are higher than the other polymers. However, most application would require much better strength and/strain at break. Presumably, the molecular weight and the weight/volume percent of the hard domain are inadequate. In addition, the bisurea segment decomposes at 150 C to result in covalent crosslink.
In prior work represented in U.S. patent application Ser. No. 14/396,951, it was demonstrated that a thermoplastic elastomer is obtained by end-functionalizing polyisobutylene with oligo(β-alanine). However, the TPE does not possess the high strength typical of thermoplastic elastomers. It is here posited the low strength is a result of the molecular weight and the weight/volume percent of the hard domain being inadequate.
In summary, elastomers with grafted or end-functionalizing, monodisperse hard segment have so far not been able to show the strength and elasticity that conventional triblock styrenic TPEs and segmented polyurethane TPEs possess. Bonuses (Gaymans, R. J. Prog. Polym. Sci. 2011, 36, 713-748) displayed by supramolecular segmented TPEs over the conventional TPEs can be expected if the strength issue can be solved.
Since many desirable hydrocarbon elastomer backbones can be used as the soft component in grafted TPEs, supramolecular grafted TPEs can be expected to occupy the market space of hydrocarbon TPEs typically different from the market of aforementioned segmented TPEs with polar polyether or polyester soft components.